We can't help if you don't disclose what is not working.
The how-to on the Lineage wiki is a s complete as it gets. What is the first step that fails in your case?
If you are using a Windows PC for flashing, it is likely that you need to install the Google OEM driver to get fastboot to work ...
I don't under stand it either. Main thing is it's working now. I have to assume that putting my SIM in the other phone, reset something, so it worked when I put it back in the Pro1.